Crystal chemistry of Ca-rich pyroxenes from undersaturated to oversaturated trachytic rocks, and their relationships with pyroxenes from basalts

A study of 23 Ca-rich pyroxenes from silica-undersaturated and -oversaturated rocks, shows that all the pyroxenes from trachytes are characterized by a relatively large M1 site, related to its high Fe2+ occupancy. The volume of the M1 site is inversely related to that of the tetrahedral site. The high Si content of the pyroxene from trachytes causes significant increase in the longest M2-O3C2,D2 distances. The most striking difference between the pyroxenes from trachytes and those from basalts is found in the volumes of the M1 and T sites, and in the degree of distortion of the M2 polyhedron (ΔM2). The relationship between the β angle and ΔM2 provides an estimate of: (1) the degree of alkalinity in trachytic melts, and (2) indications on the nature of their possible parental “basaltic” magmas.